The present invention relates to improved designs for variable coupler fiberoptic sensors and to sensing apparatus using the improved sensor designs.
Variable coupler fiberoptic sensors conventionally employ so-called biconical fused tapered couplers. Such couplers are manufactured by a draw and fuse process in which a plurality of optical fibers are stretched (drawn) and fused together at high temperature. The plastic sheathing is first removed from each of the fibers to expose the portions for forming the fusion region. These portions are juxtaposed, usually intertwisted one to several twists, and then stretched while being maintained above their softening temperature in an electric furnace or the like. As the exposed portions of the fibers are stretched, they fuse together to form a narrowed waist region—the fusion region—that is capable of coupling light between the fibers. During the stretching process, light is injected into an input end of one of the fibers and monitored at the output ends of each of the fibers to determine the coupling ratio. The coupling ratio changes with the length of the waist region, and the fibers are stretched until the desired coupling ratio is achieved—typically by a stretching amount at which the respective fiber light outputs are equal. The coupler is drawn to such an extent that, in the waist region, the core of each fiber is effectively lost and the cladding may reach a diameter near that of the former core. The cladding becomes a new “core,” and the evanescent field of the propagating light is forced outside this new core, where it envelops both fibers simultaneously and produces the energy exchange between the fibers. A detailed description and analysis of the biconical fused tapered coupler has been given by J. Bures et al. in an article entitled “Analyse d'un coupleur Bidirectional a Fibres Optiques Monomodes Fusionnes”, Applied Optics (Journal of the Optical Society of America), Vol. 22, No. 12, Jun. 15, 1983, pp. 1918-1922.
Biconical fused tapered couplers have the advantageous property that the output ratio can be changed by bending the fusion region. Because the output ratio changes in accordance with the amount of bending, such couplers can be used in virtually any sensing application involving motion that can be coupled to the fusion region.
Conventional variable coupler fiberoptic sensors have relied upon designs in which the fiberoptic coupler is pulled straight, secured under tension to a plastic support member and, in the resulting pre-tensioned linear (straight) form, encapsulated in an elastomeric material such as silicone rubber. The encapsulant forms a sensing membrane that can be deflected by external forces to cause bending of the coupler in the fusion region. The bending of the fusion region results in measurable changes in the output ratio of the coupler. The displacement of the membrane can be made sensitive to as little as one micron of movement with a range of several millimeters.
FIG. 1 of the accompanying drawings illustrates the basic principles of a sensing apparatus including a variable coupler fiberoptic sensor 10 as described above. In the form shown, the sensor 10 includes a 2×2 biconical fused tapered coupler 11 produced by drawing and fusing two optical fibers to form the waist or fusion region 13. Portions of the original fibers merging into one end of the fusion region become input fibers 12 of the sensor, whereas portions of the original fibers emerging from the opposite end of the fusion region become output fibers 14 of the sensor. Reference numbers 18 denote the optical fiber cores. The fusion region 13 is encapsulated in an elastomeric medium 15, which constitutes the sensing membrane. The support member is not shown in FIG. 1.
In practice, one of the input fibers 12 is illuminated by a source of optical energy 16, which may be an LED or a semiconductor laser, for example. The optical energy is divided by the coupler 11 and coupled to output fibers 14 in a ratio that changes in accordance with the amount of bending of the fusion region as a result of external force exerted on the sensing membrane. The changes in the division of optical energy between output fibers 14 may be measured by two photodetectors 17 which provide electrical inputs to a differential amplifier 19. Thus, the output signal of differential amplifier 19 is representative of the force exerted upon medium 15. It will be appreciated that if only one of the input fibers 12 is used to introduce light into the sensor, the other input fiber may be cut short. Alternatively, it may be retained as a backup in the event of a failure of the primary input fiber. It should be noted that, for simplicity, the coupler 11 is shown without the aforementioned fiber twisting in the fusion region. Such twisting is ordinarily preferred, however, to reduce lead sensitivity, which refers to changing of the output light division in response to movement of the input fiber(s).
Because variable coupler fiberoptic sensors can be made entirely from dielectric materials and optically coupled to remote electronics, they are particularly advantageous for applications in which the presence of electrically conductive elements at the sensor location would pose the risk of electrical shock, burns, fire, or explosion. In the medical field, for example, variable coupler fiberoptic sensors have been proposed for monitoring patient heartbeat during MRI examinations. See U.S. Pat. No. 5,074,309 to Gerdt, which discloses the use of such sensors for monitoring cardiovascular sounds including both audible and sub-audible sounds from the heart, pulse, and circulatory system of a patient. The use of sensing devices having metallic components in an MRI environment has been known to cause severe burning of patients due to the presence of strong radio frequency fields.
Other applications of variable coupler fiberoptic sensors can be found in U.S. Pat. No. 4,634,858 to Gerdt et al. (disclosing application to accelerometers), U.S. Pat. No. 5,671,191 to Gerdt (disclosing application to hydrophones), and elsewhere in the art.
As compared with other types of fiberoptic sensors, the described variable coupler sensors offer a uniquely advantageous combination of low cost, relatively simple construction, high performance (e.g., high sensitivity and wide dynamic range), and versatility of application. Other known fiberoptic sensors have used such principles as microbending loss, light phase interference, and polarization rotation by means of birefringence. Fiberoptic micro-bending sensors are designed to sense pressure by excluding light from the fiber in proportion to the changes in pressure. The output light intensity decreases with increases in measured pressure, as pressure is transduced into light loss. Because the measurement accuracy is reduced at lower light levels, the dynamic range of such sensors is severely limited. Interferometric fiberoptic sensors measure changes in pressure by applying pressure to an optical fiber to change its index of refraction. This results in a phase delay that is measured by utilizing a Mach-Zehnder or Michaelson interferometer configuration. These sensors are extremely expensive and require sophisticated modulation techniques that render them unsuitable for many applications. Polarization varying fiberoptic sensors alter the polarization state of a polarized optical signal in accordance with a change in temperature or pressure. Such polarized light sensors require special optical fiber and expensive polarizing beam splitters.
Despite their advantages, variable coupler fiberoptic sensors have been subject to certain limitations inherent in the conventional pre-tensioned linear (straight) coupler design. The conventional design imposes, among other things, significant geometrical limitations. In particular, the size of the sensor must be sufficient to accommodate the fiberoptic leads at both ends of the sensor. The fiberoptic lead arrangement also requires the presence of a clear space around both ends of the sensor in use. Especially in medical applications, such as when placing a sensor on a patient's body for continuous monitoring, the size and lead positions of the sensor are both important issues. Another limitation results from the fact that any displacement of the fusion region necessarily places it under increased tension. At some point of displacement, the tension in the fusion region will become excessive, causing the fusion region to crack or break, with resulting failure of the coupler.